Synthesis of Tyr-Pro-Phe-Val-Glu-L-[3,4-3H]Pro-Ile, Tyr-D-Ala-Phe-Gly-Tyr-L-[3,4-3H]Pro-Ser-NH2, and Tyr-D-Ala-Phe-Gly-Tyr-D-[3,4-3H]Pro-Ser-NH2, Labeled Analogs of Human β-Casomorphin and Dermorphin